Gamma irradiators are used to deliver a pre-determined amount of gamma radiation in a controlled manner to products. Controlled amounts of gamma radiation are used to sterilize products, such as single use syringes, surgical gloves, laboratory ware and other disposable medical devices. Products such as food packaging, spices and other foods are treated with lower levels of radiation to reduce microbiological contamination. Irradiators provide a mechanism to irradiate products while closely regulating the amount of radiation to which products are exposed and confining the radiation to a limited area.
Gamma irradiators commonly have a cell area in which the products are treated with radiation. Various types of walls, shields, and mazes are used to prevent radiation from leaving the cell area. The products are introduced into the cell area either manually in a batch system or on a conveyor in a continuous loading system, the radiation source is mechanically moved from its storage position into the operating position for irradiation. During processing, radiation from the raised source effectively kills organisms anywhere in or on the products which pass through the irradiator. The absorbed dose delivered to the products is a function of the time the product is exposed to the radiation source, the quantity of cobalt-60 isotope or other radiation source in the system, the distance the product is from the radiation source and the density of the product itself.
The walls, shield, maze or other structure surrounding the cell area dictates the design of the irradiator. The structure surrounding the cell area must absorb the gamma radiation after it passes through the product being irradiated to ensure that the outside of the shield maintains radiation levels required by the United States Nuclear Regulatory Commission ("USNRC") and the International Atomic Energy Agency ("IAEA") regulations (currently less than 2 millirem per hour).
Irradiators of an industrial scale, now known to the industry, use a "poured in place" steel reinforced concrete design for the structure. In order to comply with the required radiation levels outside of the cell area, these irradiators generally have wall thicknesses in excess of six feet to adequately shield radiation source capacities from 3 to 12 Megacuries. The typical size of the shielding structure of a large industrial irradiator system is from 2000 to 4000 square feet. Irradiators of this size built using poured in place concrete construction are impractical to build in existing buildings because of their large size and the requirements of standard construction techniques. In addition, irradiators constructed of standard poured in place concrete cannot be fully decommissioned and moved to other locations. In addition, all construction must take place on-site adding to the cost and construction time of the facility.
Large irradiators also have disadvantages in operation. They cannot effectively and economically process a small volume of product in geographic locations. Because standard irradiators are large and immovable, they can be built only in limited locations where volumes are large enough to justify the facility. When irradiating small quantities of product, it is not economically feasible to transport the products long distances to the location of the irradiator. In addition, the large size of the cell area of standard irradiators makes processing of small amounts of product inefficient.
The shielding structure in an industrial sized irradiator commonly includes some kind of maze design to allow access to the cell area. The product enters the cell area through the maze where the many turns in the maze prevent radiation from leaving the cell area. This maze design further increases both the size and cost of the shielding structure.
Irradiators also include a system for transporting the products into and out of the cell area and around the radiation source. This system can be a system of conveyors, which continually cycles product into and out of the cell area, or a batch system. Existing batch style irradiators of an industrial size require the operators to physically enter the cell area through the maze between batch runs to exchange the processed product for the new batch of product to be processed. Any requirement that personnel enter into the cell area inherently decreases the safety of the irradiator.
Irradiators also commonly include a source system which stores the radiation source when the irradiator is not processing product and moves the source into position for processing. The source system in standard radiators includes a rectangular pool of water below the floor of the cell area to store the source when it is not in use. The depth required to adequately shield the radiation source can make construction of the storage system impractical or prohibitively expensive in certain locations due to soil conditions or other environmental factors. In addition, the excavation required for the rectangular shape of the pool can be expensive to construct.
For the foregoing reasons, there is a need for a movable and moderately sized gamma irradiator that operates efficiently and economically.